DNA & the tracking chip.
UN scientists developed a radar-gun like tool that can detect mutated DNA. It's since been updated to also scan for the chip, so the law gets a green light (and expects your cooperation) if you have the chip, and an alarm goes off for them if you don't.
In the instance where something paranormal occurs and the people involved haven't turned into an animal, set off the gun, or proven themselves to be vampires, police officers are usually dumbfounded. Major cities tend to have a sensitive-detective to help in issues like this, but that's a relatively new practice for most countries.